This invention relates to communications networks, generally, and more particularly, to a method of communication within such communications networks and apparatus for practicing the inventive method.
A communications network provides a means for communication, i.e., exchange or transfer of information, between a multiplicity of data nodes, e.g., host computers, connected within the network by a communications subnet. The subnet comprises switching nodes connected to each other and to the hosts by transmission links capable of carrying transmitted information. Information transmitted via the communications network is often formatted into a plurality of distinct packets for transmission purposes. A data node may be categorized, for any particular communication, as a source (sender) or a destination (receiver). Each packet has a header containing all the routing information necessary to control the switching nodes in order to move the packet along the network from the source to the destination. Packets from a source are switched from switching node to switching node until the packets reach their intended destination.
Various routing protocols are currently used to control the routing of packets from node to node through a communications network. In a point-to-point communications subnet, a destination node requests information from a source node. In this arrangement, the source may be characterized as a server and the destination may be characterized as a client. In such a scenario, the client sends a request for information over the communications network to the server and the server responds by sending the information to the client. The client and the server on a communications network frequently communicate via a transmission control protocol (xe2x80x9cTCPxe2x80x9d) connection. A TCP connection is a bi-directional single communication channel. TCP is a popular method of formatting information into packets for transmission over the communications network.
Frequently, communications between clients and servers involve the transfer of datasets, namely, electronic computer files, from a server to a client. A current popular method of file transfer makes use of a protocol called file transfer protocol (FTP). An FTP session consists of a first TCP connection for control communications (xe2x80x9ccontrol TCP connectionxe2x80x9d) and a second TCP connection for data communications (xe2x80x9cdata TCP connectionxe2x80x9d). In a typical mode of FTP transmission, the file, or xe2x80x9cdatasetxe2x80x9d, to be transmitted is typically divided into a multiplicity of discrete segments or xe2x80x9cblocksxe2x80x9d by the server. The blocks are then subdivided into smaller TCP packets of digital information and transmitted to the client via the data TCP connection. Once the client has received all of the packets comprising the dataset, the FTP session is terminated. The client has the ability to receive the packets nonsequentially and then sort and reassociate the packets to replicate the blocks transmitted by the server. This ability is referred to herein as the xe2x80x9cnonsequential disk writingxe2x80x9d capability.
In a multicast communications subnet, a multicast routing protocol is used. Multicast routing is well-known in the art. In a multicast, a single source transmits a communication once and that communication is received at a multiplicity of destinations. In other words, the sender xe2x80x9cmulticastsxe2x80x9d the packets once and a multiplicity of destination hosts may xe2x80x9clistenxe2x80x9d to the multicast and receive the packets comprising the communication.
In a typical multicast routing protocol, a destination tree or xe2x80x9cmulticast treexe2x80x9d is defined. A multicast tree is characterized by a root connected to a set of intermediate switching nodes and links which end in a set of leaves. The tree is a database of electronic addresses of switching nodes and links. A typical multicast tree has a source data node as its root, a set of destination nodes as its leaves, and intermediate switching nodes and transmission links connecting the root to the leaves.
The distribution of a file by multicast is typically advertised in advance of the specified time and date of distribution. The advertisement may take many forms but will typically be displayed on a web page providing instructions for joining the multicast tree. Interested individuals will note the time and date of the multicast and listen to the multicast at the designated time and date. Users who fail to join the multicast at the appropriate time will not receive the desired file. The multicast is achieved by transmitting the file from the source to a switching node which has multicast capability and is aware of the intended multicast tree. In this way, the communication is transmitted between the source and the multicast switching node only once. The multicast switching node, on an edge of the network, then relays the communication to the clients joined to the multicast tree.
The multicast tree is identified in the routing field of the packet header by a unique multicast tree address. The same address is associated with each transmission link forming part of the multicast tree. When a multicast packet reaches a switching node, the address in the packet header is compared to the address associated with all outgoing transmission links from that switching node. The packet is then forwarded on each transmission link for which an address match occurs, except for the link on which the packet is received. The destination nodes receive the multicasted packets contemporaneously, or substantially contemporaneously, due to the fact that a packet may have to be switched through varying numbers of switching nodes before reaching various clients.
For multicasts, a modified TCP connection is used between the server and the multicast switching node. This modified TCP connection uses intermediate multicast switching nodes as proxies such that the source waits for acknowledgment of receipt of packets from the intermediate multicast switching nodes rather than from each destination to determine whether the packets have been received complete and intact. The proxies cache the TCP packets they relay until receipt from all intended destinations is acknowledged or until a predetermined length of time passes. Use of such a modified TCP connection is well-known in the art.
Such multicast tree routing has become a common method of transmission due to the efficiencies in the multicast tree protocol.
When a predefined group of clients are to receive a multicast communication, the efficiencies of multicast can be realized. Additionally, when a multiplicity of clients which are members of a multicast tree request a file from a server, and the clients"" requests are synchronized, the file may be transmitted once over the network using multicast protocol. The use of multicasts is well known in the art, particularly for computer software distribution. However, the clients must cooperate or be centrally controlled to synchronize their requests in order to realize the savings in network resources associated with the multicast procedure.
In many instances, particularly with the widespread use of the Internet and the World Wide Web, the clients are numerous, not centrally controlled, and are not members of a predefined multicast tree. Rather, the clients are autonomous. Accordingly, the clients"" requests for a particular computer file residing on a server are likely to be asynchronous. This is frequently the case for requests for transfer of a new computer software version, general stock market information, etc. The current method of serving such asynchronous requests for file transfer is to utilize separate FTP sessions between the server and the clients for each request. Accordingly, the network resource consumption grows linearly with the number of clients (2N TCP connections for N clients). Because of the limited resources of communications networks generally, and of servers, switching nodes, and transmission links, specifically, it is desirable to reduce the amount of network resources required to service a number of clients in the case of asynchronous requests for file transfers.
Accordingly, it is an object of the present invention to provide an efficient method of communication capable of accommodating asynchronous requests for file transfer from a multiplicity of autonomous clients.
It is another object of the present invention to provide an efficient method of communication which employs multicast protocol and which reduces the amount of network resources used to respond to synchronous requests of a multiplicity of autonomous clients.
It is yet another object of the present invention to provide a communications network capable of carrying out the inventive method for accommodating asynchronous requests for file transfer from a multiplicity of clients.
It is a further object of the present invention to provide a source node capable of carrying out the inventive method for accommodating asynchronous requests for file transfer from a multiplicity of clients.
It is yet a further object of the present invention to provide a destination node capable of carrying out the inventive method for accommodating asynchronous requests for file transfer from a multiplicity of clients.
These and other objects are realized by the provision of a method providing for sequential transmission of a dataset in discrete blocks over a single data connection spanning the router network. The method further provides for subsequent joining of destinations to a transmission already in progress. Each subsequently joined destination begins reception of blocks with the next sequentially transmitted block and continues to receive blocks until all blocks in the dataset have been received. Each subsequently joined destination transmits a status signal to the source to ensure retransmission of blocks transmitted prior to a first received block received at each destination.
These and other objects are also realized by the provision of a communications network comprising: a source node for transmission of a dataset in a series of blocks, the source node being capable of retransmitting a block responsively to a status signal, the source node being further capable of receiving the status signal intermediate the transmission of the series of blocks; at least a first and a second destination node connected to the network, each destination node being capable of requesting transmission of the dataset from the source node and being further capable of transmitting a status signal to the source indicating its status of reception of blocks relative to the blocks in the dataset; and a multicast switching node for relaying a block received from the source to a destination, the multicast switching node being capable of joining a destination to a transmission intermediate the transmission.
A source node and a destination node for use in carrying out the inventive method are also provided.